Alien Submarines

The Alien Submarines used by the aliens of Terror from the Deep are big, blocky, shapes which resemble sea creatures and lack most forms of aerodynamics. They are able to dive and fly at breakneck speeds through the power of Zrbite and the resiliency of Aqua Plastics. Alien Subs come in a variety of forms ranging from the tiny Survey Ships right up to the massive Dreadnoughts.

Alien Subs are the aliens' sole method to move around Earth, and collect resources, so in theory, it would be possible to halt the alien advance if X-COM could maintain absolute naval superiority. Earth's forces do not initially have the resources required for such a task, and even if it becomes possible, the aliens have an vast resource supply and will not be set back by a war of attrition. In addition, alien undercover agents will continue to erode Earth's defenses from the inside.

Despite this, commanders should still down as many Alien Subs as possible. At the very least, it may slow the aliens down some.

* Actual interval before an Alien Sub flees is calculated by randomly increasing this value by 0-100%, then subtracting 30 per difficulty level. For example, the Escort on Superhuman (difficulty 6 in the game code) will attempt to flee after 70-320 frames. The Acceleration value in the in-game UFOpedia is not used.

** The figures given in the in-game UFOpedia are a factor of 8 too high, relative to Sub Armament ranges.

*** Actual interval between Alien Sub attacks is calculated by subtracting 2 per difficulty level from this value, then randomly increasing the result by 0-100%. For example, the Dreadnought on Superhuman (difficulty 6 in the game code) will fire every 12 to 24 frames (average 18).

If your craft is faster, the Alien Sub cannot escape; if your weapons have a higher range than the Alien Sub's, you'll be entirely safe from return fire.

Sonic Oscillators can be used (even by Barracudas) with impunity against any Alien Submarines except Dreadnoughts. Even against Dreadnoughts, Sonic Oscillators are better than P.W.T. Launchers because a minimum of four P.W.T.-armed craft are required to down a Dreadnought.

Single craft with dual DUPs (expected total damage = 396) are sufficient to bring down any Small or Medium Alien Subs. 3 craft with dual DUPs are needed to defeat a Fleet Supply Cruiser. DUPs cannot be used safely against Battleships (unlike UFO's Avalanche, which outranges Terror Ships).

A single craft armed with dual Gauss Cannon (expected total damage = 2362.5) could expect reliably to bring down any craft, including a Dreadnought - provided it can survive long enough.

A single craft armed with humble dual Gas Cannon (expected total damage = 1125) could also expect reliably to bring down any craft except a Dreadnought - again, provided it can survive long enough. (It will need to survive 40% longer than a Gauss Cannon-equipped craft, as the Gauss Cannon has 40% more firepower).

X-Com Subs vs. Alien Subs

The Barracuda can always catch Survey Ships for the Barracuda is faster, but it often cannot dive deep enough to engage its target.

The Manta is faster than all Alien Subs EXCEPT the Dreadnought.

The Hammerhead can intercept all Alien Subs except the Hunter, the Battleship, and the Dreadnought, which must slow down in order for the Hammerhead to catch up.

The Leviathan is 1000 speed units faster than even the mighty Dreadnought and can intercept all Alien Subs.

Quirks

The floor maps & schematic pictures of the Survey Ship and Escort are swapped with each other in the Battlescape, the Interception window and the UFOPaedia.

The schematic pictures of the Battle Ship and the Fleet Supply Cruiser are also swapped in the UFOPaedia and Intercept window. The floorplan maps on the Battlescape are not affected.